1. Field of the Invention
The present invention relates to a video signal processing apparatus having a digital multivalue synchronizing signal generating circuit, used for the purpose of obtaining a video signal having a analog synchronizing signal in high definition television systems.
2. Description of the Prior Art
The Hi-Vision system is known as one of the high definition television systems. In the Hi-Vision system, aside from digital signal processing within the system, connection of systems may be achieved by transmission of analog video signal. Among the analog video signal, in the wide band signal called the base band signal, three signals, Y (luminance signal), PB (color difference signal) and PR (color difference signal), or three signals, G (green signal), B (blue signal) and R (red signal), are used. Further, generally, three-value synchronizing signals of both positive and negative polarity are added to these analog video signals (reference: Broadcasting Technology Association Standard BTAS-001, August 1987).
In the conventional system, as shown in FIG. 5, a digital video signal is converted into an analog video signal by a video D/A converter 51 and a video LPF (low pass filter) 52, and a digital synchronizing signal generated in a synchronizing signal generator 53 is converted into an analog synchronizing signal in a synchronizing D/A converter 54, filtered In a synchronizing LPF 55, and added by analog signal processing in an adder 56, and a video signal with an analog synchronizing signal is obtained.
Today's signal processing systems require a reduced size and higher performance making circuit, digitization and integration necessary. In the case of The Hi-Vision system in which signal processing within the system is digital, digitization is also desired in the signal processing unit for adding a synchronizing signal to the video signal. It is hence considered to obtain a video signal having a digital synchronizing signal by combining a digital video signal and a digital synchronizing signal, and to convert the same into a video signal having an analog synchronizing signal using a single D/A converter, and filtered using a single LPF.
This LPF must have a flat characteristic in order to prevent damping of the video signal In the passing band and a steep damping characteristic for removing higher harmonics in the blocking band. The digital synchronizing signal generated in a conventional synchronizing signal generator has an infinite frequency band theoretically. Therefore, when a digital video signal and the conventional digital synchronizing signal are directly combined and D/A converted, and the video signal is limited in band by using a proper LPF, a transient response called ringing occurs in the synchronizing signal portion. In order to obtain a smooth synchronizing signal waveform by improving ringing, it is necessary to use a digital multivalue synchronizing signal having an equivalent frequency band to the digital video signal. A large number of memories is required for generating one frame portion of c digital multivalue synchronizing signal having an equivalent frequency band to the digital video signal, making it difficult to realize such a circuit.